Thread: bulb problem

A light bulb uses 1.2 A when connected across a 120 V source. What power is used by the bulb?

Also, what current flows through a 75W bulb connected to a 120 V outlet?

thanks for any help with this question.

Originally Posted by rcmango

A light bulb uses 1.2 A when connected across a 120 V source. What power is used by the bulb?

the power, P = I^2 * R = (V^2)/R
but what is R? we can find it using ohms law

V = IR
=> R = V/I = 120/1.2 = 100

=> P = (V^2)/R = (120^2)/100 = 144 watts

Also, what current flows through a 75W bulb connected to a 120 V outlet?

P = (V^2)/R
=> 75 = (120^2)/R
=> R = (120^2)/75 = 192 ohms

Now V = IR
=> I = V/R = 120/192 = 0.625 amps

or we could have found I directly, without first finding R:

P = (I^2)*R
since V = IR => R = V/I
so P= (I^2)(V/I) = IV
=> I = P/V = 75/120 = 0.625

thanks for breaking this one down for me.

Originally Posted by rcmango

A light bulb uses 1.2 A when connected across a 120 V source. What power is used by the bulb?

Also, what current flows through a 75W bulb connected to a 120 V outlet?

thanks for any help with this question.

P=IV

RonL